Methods and system for performing handover in a wireless communication system

ABSTRACT

A method and system for performing handover in a third generation (3G) long term evolution (LTE) system are disclosed. A source evolved Node-B (eNode-B) makes a handover decision based on measurements and sends a handover request to a target eNode-B. The target eNode-B sends a handover response to the source eNode-B indicating that a handover should commence. The source eNode-B then sends a handover command to a wireless transmit/receive unit (WTRU). The handover command includes at least one of reconfiguration information, information regarding timing adjustment, relative timing difference between the source eNode-B and the target eNode-B, information regarding an initial scheduling procedure at the target eNode-B, and measurement information for the target eNode-B. The WTRU then accesses the target eNode-B and exchanges layer 1/2 signaling to perform downlink synchronization, timing adjustment, and uplink and downlink resource assignment based on information included in the handover command.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/795,242, filed Jul. 9, 2015, which is a continuation of U.S. patentapplication Ser. No. 14/100,286, filed Dec. 9, 2013, which issued asU.S. Pat. No. 9,113,374 on Aug. 18, 2015, which is a continuation of theU.S. patent application Ser. No. 13/359,072, filed Jan. 26, 2012, whichissued as U.S. Pat. No. 8,886,191 on Nov. 11, 2014, which is acontinuation of U.S. patent application Ser. No. 11/765,013, filed Jun.19, 2007, which issued as U.S. Pat. No. 8,131,295 on Mar. 6, 2012, whichclaims the benefit of U.S. Provisional Patent Application No. 60/815,023filed Jun. 20, 2006, the contents of which are hereby incorporated byreference herein.

FIELD OF THE INVENTION

The present invention is related to wireless communication systems. Moreparticularly, the present invention is related to a method and systemfor performing handover in a long term evolution (LTE) system.

BACKGROUND

LTE for the fourth generation (4G) system is now being considered todevelop anew radio interface and radio network architecture thatprovides a high data rate, low latency, packet optimization, andimproved system capacity and coverage. For an LTE system, instead ofusing code division multiple access (CDMA), which is currently beingused in a 3G system, orthogonal frequency division multiple access(OFDMA) and frequency division multiple access (FDMA) are proposed to beused in downlink and uplink transmissions, respectively. By changing inmany aspects in the LTE system, intra-LTE handover procedures andrelated operations need to be re-considered.

The user equipment (UE) mobility management in an LTE_ACTIVE modehandles all necessary steps for seamless handover in the LTE system,such as making an intra-LTE handover decision on a source network side,(i.e., control and evaluation of UE and evolved Node-B (eNode-B)measurements taking into account UE-specific area restrictions),preparing radio resources on a target network side, commanding the UE tointerface with new radio resources, releasing radio resources on thesource network side, and the like. The UE mobility management mechanismalso handles the transfer of context data between involved nodes, andthe update of node relations on a control plane (C-plane) and a userplane (U-plane).

FIG. 1 is a signaling diagram of a handover process 100 currentlyproposed for the LTE system. A UE 152 and a source eNode-B 154 performmeasurements and exchange measurement reports (step 102). The sourceeNode-B 154 makes a handover decision based on the measurement reports(step 104). The source eNode-B 154 then sends a handover request to atarget eNode-B 156 (step 106). The handover decision and subsequentprocedures before handover completion are performed without involving amobility management entity/user plane entity (MME/UPE) 158, (i.e.,handover preparation messages are directly exchanged between the sourceeNode-B 154 and the target eNode-B 156).

The target eNode-B 156 performs an admission control for the UE 152(step 108). If the target eNode-B 156 can accept the UE 152, the targeteNode-B 156 sends a handover response to the source eNode-B 154 (step110). The source eNode-B 154 sends a handover command to the UE 152(step 112). For seamless handover, a U-plane tunnel is establishedbetween the source eNode-B 154 and the target eNode-B 156.

The UE 152 and the target eNode-B 156 then exchange layer 1 and 2(L1/L2) signaling (step 114). During handover execution, user data maybe forwarded from the source eNode-B 154 to the target eNode-B 156. Theforwarding may take place in a service dependent and implementationspecific way. Forwarding of user data from the source eNode-B 154 to thetarget eNode-B 156 should take place as long as packets are received atthe source eNode-B 154 from the UPE 158.

After a connection to the target eNode-B 156 is established, the UE 152sends a handover complete message to the target eNode-B 156 (step 116).The target eNode-B 156 sends a handover complete message to the MME/UPE158 (step 118). The MME/UPE 158 then sends a handover completeacknowledgement (ACK) to the target eNode-B 156 (step 120). After theMME/UPE 158 is informed by the target eNode-B 156 that the UE 152 hasgained an access at the target eNode-B 156 by the handover completemessage, the U-plane path is switched by the MME/UPE 158 from the sourceeNode-B 154 to the target eNode-B 156.

The release of the radio resources at the source eNode-B 154 istriggered by a release resource message sent by the target eNode-B 156(step 122). After receiving the release resource message from the targeteNode-B 156, the source eNode-B 154 releases the radio resources for theUE 152 (step 124). The UE 152 performs a location update with theMME/UPE 158 (step 126).

The above intra-LTE handover procedure 100 does not provide detailsregarding the handover command, (such as configurations of the UE 152based on the target eNode-B's requirement), and details regarding UEoperation after the UE receives the handover command, (such as datatransmission between the source eNode-B 154 and the UE 152 and radiolink control (RLC) and hybrid automatic repeat request (HARQ) reset andpacket data convergence protocol (PDCP) sequence number (SN) gapidentification by the UE 152). The above intra-LTE handover procedure100 also does not provide details regarding UE timing adjustment forsynchronous and asynchronous eNode-Bs and details for efficient targeteNode-B scheduling of resources for UE transmission.

SUMMARY

A method is provided for recovering from an unsuccessful handover. Themethod may include a wireless transmit/receive unit (WTRU) determiningthat a procedure for handover of the WTRU from a source cell to a targetcell was unsuccessful. The method may also include the WTRU initiating aradio link failure procedure in response to determining that theprocedure for handover was unsuccessful. The WTRU may send an indicationof a cell identity (ID) of the source cell with a radio networktemporary identifier (RNTI) during the radio link failure procedure. TheWTRU may attempt to access at least one cell upon determining that theprocedure for handover was unsuccessful. The at least one cell mayinclude the source cell, a second cell being served by the same evolvedNode-B (eNode-B) as the source cell, and/or a third cell being served bya different eNode-B than the eNode-B serving the source cell. The thirdcell may be selected based on a measurement result. The RNTI may be anRNTI for a Long Term Evolution (LTE) system.

An eNode-B may include a transceiver configured to transmit and receivedata, and a controller, coupled to the transceiver. The controller maybe configured to send a handover command to a WTRU. The handover commandmay instruct the WTRU to handover over from a source cell served by theeNode-B to a target cell. The controller may be configured to receive anindication of a cell identity (ID) of the source cell with a RNTI duringa radio link failure procedure. The controller may be further configuredto inform a target eNode-B of an unsuccessful handover of the WTRU fromthe source cell to a target cell of the target eNode-B. The controllermay be further configured to maintain a timer. Expiration of the timerprior to receipt of a handover complete message may indicate a handoverfailure. The controller may be further configured to reset radio linkcontrol (RLC) parameters and hybrid automatic repeat request (HARQ)parameters based on expiration of the timer.

The present invention is related to a method and system for performinghandover in an LTE system. A source eNode-B makes a handover decisionbased on measurements, and sends a handover request to a target eNode-B.The target eNode-B sends a handover response to the source eNode-Bindicating that a handover should commence. The source eNode-B thensends a handover command to a wireless transmit/receive unit (WTRU). Thehandover command includes at least one of reconfiguration information,information regarding timing adjustment, relative timing differencebetween the source eNode-B and the target eNode-B, information regardingan initial scheduling process at the target eNode-B, and measurementinformation for the target eNode-B. The WTRU then accesses the targeteNode-B and exchanges layer 1/2 signaling to perform downlinksynchronization, timing adjustment, and uplink and downlink resourceassignment based on information included in the handover command.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred embodiment, given by way of exampleand to be understood in conjunction with the accompanying drawingswherein:

FIG. 1 is a signaling diagram of a handover process currently proposedfor the LTE system; and

FIG. 2 is a signaling diagram of an intra-LTE handover process inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When referred to hereafter, the terminology “WTRU” includes but is notlimited to a UE, a mobile station, a fixed or mobile subscriber unit, apager, a cellular telephone, a personal digital assistant (PDA), acomputer, or any other type of user device capable of operating in awireless environment. When referred to hereafter, the terminology“eNode-B” includes but is not limited to a base station, Node-B, a sitecontroller, an access point (AP), or any other type of interfacingdevice capable of operating in a wireless environment.

The present invention provides detailed procedures for signaling andoperations at a WTRU and source and target eNode-Bs during intra-LTEhandover both for successful handover and handover failure cases. In asuccessful handover case, new information elements (IEs) are added inboth the handover command message and the handover complete message. Ina handover failure case, new signaling messages are exchanged between asource eNode-B and a target eNode-B.

FIG. 2 is a signaling diagram of an intra-LTE handover process 200 inaccordance with the present invention. A WTRU 252 and a source eNode-B254 each perform at least one measurement, and the WTRU 252 sends ameasurement report to the source eNode-B 254 (step 202). The sourceeNode-B 254 makes a handover decision based on the measurement reportand the result of its own measurement (step 204). The source eNode-B 254then sends a handover request to a target eNode-B 256 (step 206). Thetarget eNode-B 256 performs an admission control for the WTRU 252 (step208). If the target eNode-B 256 can accept the WTRU 252, the targeteNode-B 256 sends a handover response to the source eNode-B 254indicating that a handover should commence (step 210). The sourceeNode-B 254 then sends a handover command to the WTRU 252 (step 212).

The handover command should include at least one of reconfigurationinformation for radio resource control (RRC), radio link control (RLC),medium access control (MAC) and physical (PHY) layer, informationregarding timing adjustment when handing over from the source eNode-B254 to the target eNode-B 256, (i.e., whether the WTRU 252 shouldperform timing adjustment autonomously or using a random access channel(RACH) procedure, if a RACH is to be used, whether random or dedicatedaccess signature will be used, or the like), relative timing differencebetween eNode-Bs (or cells) for autonomous timing adjustment,information regarding initial radio resource scheduling procedure at thetarget eNode-B 256, measurement information for the target eNode-B 256,and the like. The information regarding the initial scheduling procedureat the target eNode-B 256 indicates whether a RACH access procedureshould be used for a resource assignment request or the target eNode-B256 may schedule resources for the WTRU 252 without receiving anexplicit resource assignment request from the WTRU 252. Alternatively,the measurement and other configuration information may be sent to theWTRU 252 by the target eNode-B 256 after receiving a handover completemessage from the WTRU 252 at step 226.

For a seamless handover, a U-plane tunnel is established between thesource eNode-B 254 and the target eNode-B 256. After sending thehandover command, the source eNode-B 254 may forward the user data tothe target eNode-B 256. The forwarding may take place in a servicedependent and implementation specific way.

After receiving the handover command from the source eNode-B 254, theWTRU 252 may continue to transmit and receive data to and from thesource eNode-B 254. The data transmission process depends on whethersynchronized handover or non-synchronized handover is used.

When a synchronized handover procedure is used, (i.e., the sourceeNode-B 254 and the target eNode-B 256 are synchronized or the relativetiming difference is known to the WTRU 252), the source eNode-B 254 andthe WTRU 252 may continue to transmit and receive data after receivingthe handover command until a certain handover time (t_(HO)) which issignaled via the handover command. The transmitted data after receivingthe handover command is preferably limited to incomplete service dataunits (SDUs), (i.e., RLC protocol data unit (PDU)), transmitted beforethe handover command was sent. An RLC control message is sent to theWTRU 252 to indicate a sequence number (SN) of a successfully receivedSDU(s) and an SDU gap. The SN may be a PDCP SN, or other types of SN. AnSN common to the successfully received SDU(s) and unsuccessfullyreceived SDU(s) may be included in the RLC control message.

When a non-synchronized handover procedure is used, (i.e., the sourceeNode-B 254 and the target eNode-B 256 are not synchronized or therelative timing difference is not known to the WTRU 252), the sourceeNode-B 254 stops transmission as soon as the source eNode-B 254 sendsthe handover command to the WTRU 252. The WTRU 252 also stopstransmission of the data packets to the source eNode-B 254 as soon asthe WTRU 252 receives the handover command. Alternatively, the sourceeNode-B 254 may continue transmission of data packets until the WTRU 252switches to the target eNode-B 254.

After receiving the handover command, the WTRU 252 accesses the targeteNode-B 256 and exchange layer 1/2 (L1/L2) signaling with the targeteNode-B 256 to perform downlink synchronization, timing adjustment,(i.e., uplink synchronization), and uplink and downlink resourceassignment based on information included in the handover command.

For timing adjustment, (i.e., uplink synchronization), the WTRU 252implements one of two options. Preferably, the network decides whichoption to be used.

In accordance with a first option, the WTRU 252 autonomously performsthe timing adjustment based on relative timing difference between thesource eNode-B 254 (or cell) and the target eNode-B 256 (or cells) (step214 a). The relative timing difference information is preferablyincluded in the handover command.

In accordance with a second option, a conventional RACH access procedureis used for the timing adjustment (step 214 b). The WTRU sends a RACHpreamble to the target eNode-B and the target eNode-B calculates timingoffset based on the transmitted RACH preamble and sends the timingoffset information to the WTRU for uplink synchronization.

A plurality of RACH preamble signatures with different orthogonality anddifferent priority may be used, and among the plurality of RACH preamblesignatures, a RACH preamble signature with higher orthogonality, higherpriority and/or higher power may be used for the handover purpose.

A particular (dedicated) RACH preamble signature may be reserved for thehandover purpose to indicate that the sender is a handover WTRU, (i.e.,a WTRU undergoing a handover process). This dedicated RACH preamblesignature is indicated in the handover command. After receiving thereserved RACH preamble signature, the target eNode-B 256 recognizes thatthe sender is a handover WTRU and may provide a priority to the handoverWTRU. This can avoid the random access process which causes a longinterruption time during handover. Alternatively, a RACH messagefollowing the RACH preamble may explicitly indicate that the sender is ahandover WTRU. A handover WTRU is preferably given a higher priority toaccess an eNode-B (cell) than a non-handover WTRU due to statetransition. The RACH procedure using the reserved RACH preamblesignature may be used in either synchronized or non-synchronized eNode-B(or cell) handover. A physical radio resource allocation for sending thereserved RACH preamble signature to the target eNode-B 256 may also beincluded in the handover command to reduce a delay for the randomaccess.

The random access procedure may be used for different purposes. Therandom access procedure may be used to initiate communication between aWTRU and a network which requires a state transit from an LTE_idle stateto an LTE_active state. The random access procedure may be used fortiming adjustment during handover and then for an access request to thenew cell. When the random access procedure is used during handover, thedelay caused by the random access procedure should be minimized.Therefore, there should be differences, (e.g., giving a priority to ahandover WTRU), between the random access to the target eNode-B (cell)during handover and the random access to the source eNode-B (cell) in anon-handover situation because of state transition from an LTE-Idlestate to an LTE-Active state in the non-handover case.

After receiving the RACH preamble signature from the WTRU, the targeteNode B estimates the timing adjustment value and sends this value backto the WTRU (step 216).

After performing timing adjustment, (either autonomously or via a RACHpreamble transmission), the WTRU 202 may send a radio resourceassignment request to the target eNode-B 256 (step 218). The request ispreferably sent via a RACH message following the RACH preamble. Thetarget eNode-B 256 then schedules downlink and uplink resources for theWTRU 252 (step 220). Alternatively, the target eNode-B 256 may scheduleresources for the WTRU 252 without receiving an explicit request fromthe WTRU 252. The resource scheduling may take place any time after thetarget eNode-B 256 admits the WTRU at step 208. For example, for thesynchronized handover procedure, the target eNode-B 256 may schedule theuplink and downlink resources after some pre-defined time (earlier thanthe expected time for eNode-B switching).

The target eNode-B 256 sends an uplink resource assignment to the WTRU252 (step 222). This uplink resource is used for sending a handovercomplete message at step 226, not for data transmission. The WTRU 252preferably resets RLC and HARQ parameters after receiving the uplinkresource assignment from the target eNode-B 256 (step 224).Alternatively, the WTRU 252 may reset the RLC and HARQ parameters afterreceiving and processing the handover command at step 212. Theseparameters related to transmission to the target eNode-B 256 (or cell)are included in the handover command.

The WTRU 252 sends a handover complete message to the target eNode-B 256(step 226). The WTRU 252 preferably includes a starting uplink PDCP SNto be transmitted in the handover complete message. Optionally, the WTRU252 may send an RLC control message to the target eNode-B 256 after thehandover complete message to indicate the successfully transmitted SDUsand an SDU gap.

The target eNode-B 256 sends uplink and downlink resource schedulinginformation for data transmission and an RRC message to the WTRU (step228). The RRC message includes at least one of radio access bearer (RAB)reconfiguration information, a starting PDCP SN in the downlink, an RLCcontrol message, and measurement related information. Some or all of theabove information may optionally be sent as part of the handover commandor the first packet from the target eNode-B 256.

The target eNode-B 256 sends a handover complete message to the MME/UPE258 to inform that the WTRU 252 has gained an access at the targeteNode-B 256 (step 230). The MME/UPE 258 then sends a handover completeacknowledgement (ACK) to the target eNode-B 256 and switches the U-planedata path from the source eNode-B 254 to the target eNode-B 256 (step232). A release of the radio resources at the source eNode-B 254 istriggered by a release resource message sent by the target eNode-B 256(step 234). After receiving the message from the target eNode-B 256, thesource eNode-B 254 releases the radio resources for the WTRU 252 (step236).

A handover failure case is explained hereinafter by referring to FIG. 2.When the WTRU 252 is not able to handover successfully, the WTRU 252 mayresort to a radio link (RL) failure or a cell reselection procedure. Ifthe handover command fails at step 212, the source eNode-B 254 informsthe target eNode-B 256 of such a failure. The target eNode-B 256schedules any uplink and downlink resources to the WTRU 252 after step208. When performing cell reselection in a handover failure case, theWTRU 252 may first try to access the originally connected cell withinthe source eNode-B 254. If this fails, the WTRU 252 may try to accessother cells within the source eNode-B. If this also fails, then the WTRU252 may try to access to other cells not included in the source eNode-Bbased on the measurement result.

The source eNode-B 254 maintains a timer to time out if the handovercomplete message is not received after a predetermined time after thehandover command failure. The source eNode-B 254 may reset RRC context,PDCP context, RLC and HARQ parameters related to the WTRU 252 if thehandover failure timer expires. The source eNode-B then releases theradio resources for the WTRU 252.

When cell reselection is performed by the WTRU 252, the source cell oreNode-B identity (ID) is sent by the WTRU 252 to any eNode-B as part ofthe LTE-radio network temporary identity (RNTI) information for thedetection if the WTRU 252 accesses the original cell or any other cells.At the source eNode-B, the source eNode-B's MAC layer informs its RRClayer of the handover failure if the MAC layer detects failedtransmission of handover command.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention. Themethods or flow charts provided in the present invention may beimplemented in a computer program, software, or firmware tangiblyembodied in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) module.

1. A wireless transmit/receive unit (WTRU), comprising: a processorconfigured to: receive a handover command from a first base stationassociated with a source cell, wherein the handover command instructsthe WTRU to switch to communicating via a target cell associated with asecond base station; switch from communicating via the source cell tocommunicating via the target cell; and transmit a message to the secondbase station in response to switching from communicating via the sourcecell to communicating via the target cell, wherein the message comprisesinformation regarding one or more missing service data units (SDUs) andone or more received SDUs, and wherein a packet data convergenceprotocol, PDCP, sequence number, SN, is included in the information toindicate a missing SDU.
 2. The WTRU of claim 1, wherein the one or moremissing SDUs correspond to an SDU gap.
 3. The WTRU of claim 2, whereinthe message includes a radio link control, RLC, control message.
 4. TheWTRU of claim 1, wherein the handover command comprises reconfigurationinformation to be applied by the WTRU, and the reconfigurationinformation comprises one or more of radio resource control (RRC)configuration information, radio link control (RLC) configurationinformation, medium access control (MAC) configuration information, orphysical layer (PHY) configuration information.
 5. The WTRU of claim 1,wherein the handover command further comprises information regardingtiming adjustment, and the information regarding timing adjustmentcomprises random access channel (RACH) information for performing timingadjustment in the target cell.
 6. The WTRU of claim 5, wherein theprocessor is further configured to perform the timing adjustment usingan asynchronous RACH procedure.
 7. The WTRU of claim 1, wherein thehandover command includes radio bearer configuration information for aradio bearer associated with the WTRU.
 8. The WTRU of claim 7, whereinthe radio bearer configuration information includes packet dataconvergence protocol (PDCP) information.
 9. The WTRU of claim 1, whereinthe handover command includes information regarding a measurementconfiguration to be applied by the WTRU when communicating via thetarget cell.
 10. The WTRU of claim 1, wherein the processor is furtherconfigured to transmit a handover complete message to the target cell.11. A method implemented by a wireless transmit/receive unit (WTRU), themethod comprising: receiving a handover command from a first basestation associated with a source cell, wherein the handover commandinstructs the WTRU to switch to communicating via a target cellassociated with a second base station; switching from communicating viathe source cell to communicating via the target cell; and transmitting amessage to the second base station in response to switching fromcommunicating via the source cell to communicating via the target cell,wherein the message comprises information regarding one or more missingservice data units (SDUs) and one or more received SDUs, and wherein apacket data convergence protocol, PDCP, sequence number, SN, is includedin the information to indicate a missing SDU.
 12. The method of claim11, wherein the one or more missing SDUs correspond to an SDU gap. 13.The method of claim 12, wherein the message includes a radio linkcontrol, RLC, control message.
 14. The method of claim 11, wherein thehandover command comprises reconfiguration information to be applied bythe WTRU, and the reconfiguration information comprises one or more ofradio resource control (RRC) configuration information, radio linkcontrol (RLC) configuration information, medium access control (MAC)configuration information, or physical layer (PHY) configurationinformation.
 15. The method of claim 11, wherein the handover commandfurther comprises information regarding timing adjustment, and theinformation regarding timing adjustment comprises random access channel(RACH) information for performing timing adjustment in the target cell.16. The method of claim 15, further comprising performing the timingadjustment using an asynchronous RACH procedure.
 17. The method of claim11, wherein the handover command includes radio bearer configurationinformation for a radio bearer associated with the WTRU.
 18. The methodof claim 17, wherein the radio bearer configuration information includespacket data convergence protocol (PDCP) information.
 19. The method ofclaim 11, wherein the handover command includes information regarding ameasurement configuration to be applied by the WTRU when communicatingvia the target cell.
 20. The method of claim 11, further comprisingtransmitting a handover complete message to the target cell.